Gear pumps usually comprise two gears running in mesh with each other and arranged in a housing coming sealingly against the gears except at the gear meshing zones, the pump inlet and outlet being placed at the respective gear meshing zone.
A disadvantage with such pumps is that they deliver a pulsating flow. Furthermore, such pumps have not been able to be modified for use as supercharging compressors.
Screw pumps can be used to avoid the drawbacks just mentioned. Screw pumps can comprise two screws running in mesh with each other, the screw pair being radially, sealingly surrounded by a housing so that the pumped liquid flows "axially", i.e. along the "tooth spaces" radially sealed by the housing. The "teeth" of the screws usually extend more than one revolution round the screw body so that each "tooth" always has (for the whole of the screw revolution) at least one, and usually two, points of intermesh with its associated "tooth space".
Such screw machines are expensive to produce and do not provide complete sealing between suction and pressure sides. The high cost of the screw machine is, i.a. dependent on the difficulty of manufacturing the screw, and that the screws are usually made with different profiles.
Screw machines can be modified so that they function as supercharging compressors, e.g. by the screws being formed with diminishing pitch towards the outlet (so-called Lysholm compressors). It will however be appreciated that such modification further increases the cost of the screw machine, added to which it is usually necessary to arrange external synchronisation of the screws.